The present invention relates generally to CT imaging, and specifically to methods for contrast enhancement in CT imaging.
Contrast enhancement in CT imaging, by injection of contrast media into the bloodstream of a subject is well known in the art. A bolus of a suitable contrast medium, for example, comprising iodine, is injected intravenously into the subject""s body and is carried by the blood flow to an organ of interest, such as the liver. Features of the organ, such as tumors, which may ordinarily be difficult or impossible to discern, preferentially take up the contrast medium from the blood. After a certain delay, to allow the medium to reach the organ of interest in sufficient concentration, a CT scan of the subject is performed. If the scan is performed at the appropriate time, the contrast-enhanced features of the organ are clearly seen in the resultant CT images.
Generally, in performing such contrast-enhanced CT scans, a standard delay is allowed between the beginning of injection and the beginning of diagnostic scanning, for example 60-70 seconds in liver scans. The time required for the contrast medium to reach sufficient concentration in the organ, however, varies substantially from patient to patient. These variations are the result of differences in cardiac output and other circulatory parameters, and are very difficult to predict. Furthermore, after the concentration has reached its desired value, giving a high contrast for imaging, the medium begins to wash out of the organ, and contrast drops off. Therefore, when a standard scanning delay is used for all patients, a relatively large dose of contrast medium or a relatively long CT scanning period, with consequently increased radiation dosage, must be used in order to ensure that an image of sufficient contrast is captured.
General Electric Medical Systems (Milwaukee, Wis.) has introduced an optional modification to CT scanners of its manufacture known as xe2x80x9cSmartPrep,xe2x80x9d which is intended to provide a patient-dependent variable delay between bolus injection and the beginning of CT scanning. This modification is described in an article by Silverman, et al., entitled xe2x80x9cOptimal Contrast Enhancement of the Liver Using Helical (Spiral) CT: Value of SmartPrep,xe2x80x9d in the American Journal of Radiology, vol. 164 (1995), pages 1169-1171, which is incorporated herein by reference.
In SmartPrep, an initial scan is performed to produce a CT image of a slice through an organ or area of interest the patient""s body. A user, generally a physician, marks up to three regions of interest in the image, for example, the aorta, portal vein and hepatic parenchyma, if the liver is the object of the procedure. Injection of the contrast medium is begun, and starting a short time thereafter, a sequence of CT scans are performed of the same slice, preferably at a reduced level of irradiation. Images from these scans are reconstructed, and the CT values, in Hounsfield units (H), in the images for each of the regions of interest are compared with baseline values from the initial scan. When the CT value in one of the regions, preferably the aorta, reaches a desired threshold over the corresponding baseline, the sequence of scans is terminated. After a delay of approximately 10 sec. a diagnostic helical scan over the organ or area is initiated.
SmartPrep thus allows the beginning of the diagnostic scan to be cued according to the time it takes the contrast medium to reach an area in the image slice in sufficient concentration to produce strong image contrast. According to the above-mentioned article, SmartPrep is effective in optimizing the delay from the bolus injection to the beginning of diagnostic scanning, such that, for example, in a sample of 75 patients studied by Silverman and co-workers, delay times ranged from 57 to 86 sec, as against the standard 60-70 sec delay. These delays are typical for contrast-enhanced imaging of the hepatic parenchyma, which normally receives the contrast medium through the portal vein with a lag of a number of seconds relative to the aorta.
SmartPrep may not be effective, however, for imaging features and areas of organs that receive the contrast medium with only a short lag behind the appearance of the medium in the aorta, before there has been a substantial amount of perfusion through the body. Such features include arterial lesions in the liver and pulmonary areas. The delays inherent in reconstructing the images from the sequence of CT scans, and then waiting until the threshold is reached before terminating this sequence, are typically greater than the short time that it takes the contrast medium to reach the organ.
It is an object of the present invention to provide an improved method for contrast-enhanced CT scanning, by predicting when a bolus of contrast medium will reach a desired concentration level in an area of suspected pathology in the body.
In preferred embodiments of the present invention, a patient is positioned in a CT scanner, and X-ray attenuation data are acquired with respect to a slice through an area of suspected pathology in the patient""s body. The data are preprocessed, filtered and back-projected, as is known in the art, to reconstruct a reference CT image of the slice. One or more regions of interest (ROIs) are identified within this slice, wherein preferably, one of the ROIs includes the aorta or other major artery feeding a site of the suspected pathology, and another ROI includes the site itself. These ROIs are reprojected onto the preprocessed attenuation data, so as to define segments within the data corresponding respectively to each of the ROIs.
A bolus of a contrast medium, for example, comprising iodine, is injected into the patient""s bloodstream, and a CT scan of the slice, preferably a continuous dynamic scan, is initiated. During this scan, attenuation data from the ROI-related segments are acquired and preprocessed continuously. Preferably, the preprocessed attenuation data are tracked so as to generate one or more functional curves describing the increase in attenuation over time, due to influx of the contrast medium into the ROIs. The shapes of the initial portions of these curves are used to predict when the attenuation at the site of the suspected pathology will reach a predetermined value or peak. Based on this prediction, the continuous dynamic scan is terminated, and a helical scan is initiated, preferably automatically, to acquire a diagnostic, contrast-enhanced image of the area.
Preferably, the segments defined within the preprocessed attenuation data comprise segments of data acquired with respect to each ROI from a plurality of different angular views. As the CT scanner scans through these different views in succession, the preprocessed attenuation data acquired in each view are used in turn in generating the functional curves. Preferably, the data acquired in different views are combined, for example, by correlation, to increase the sensitivity of detection of increases in attenuation.
Alternatively or additionally, the attenuation data from the ROI-related segments may be back-projected to reconstruct images of one or more of the ROIs, as described in a PCT patent application filed on even date with the present application and entitled xe2x80x9cREAL TIME DYNAMIC IMAGE CONSTRUCTIONxe2x80x9d and which is assigned to the assignee of the present patent application, and whose disclosure is incorporated herein by reference. The CT values in the ROIs may be used to generate the functional curves, which are used in controlling the scanner as described herein.
Preferably, the one or more functional curves are displayed by the scanner, for example, on a video screen, as they are generated. This display allows a user to view, substantially in real time, the progress of the contrast medium entering the subject""s body. If the user observes that the curves do not show a normal increase in attenuation, he or she may intervene, for example, to initiate the diagnostic scan independent of the prediction or to terminate the scan, as appropriate.
CT images of the slice may be reconstructed intermittently while the attenuation tracking is going on. However, unlike methods of bolus tracking known in the art, such as the above-mentioned SmartPrep method, the present invention enables the progress of the bolus to be predicted without reconstructing the full slice image. The progress of the bolus may be predicted using only the preprocessed attenuation data, independent of and without the need for any image reconstruction, or by reconstructing only the ROI portion of the CT image. Therefore, the present invention allows much more rapid and precise prediction of the time at which the attenuation at the site of the suspected pathology will reach a desired value. Typically, in preferred embodiments of the present invention, the prediction is completed within a few seconds, for example seconds, so that the diagnostic scan can begin when the bolus reaches the region of interest, for example about 10 seconds after the attenuation at the aorta begins to rise. Therefore, unlike methods known in the art, the present invention may also be used in diagnostic scanning of arterial phase lesions. The rapid prediction afforded by the present invention may also be useful in reducing the dosages of contrast medium and radiation that are administered to the subject.
Although preferred embodiments are described herein with reference to certain combinations of axial and helical scans by the CT scanner, it will be appreciated that the principles of the present invention, whereby the attenuation data themselves are used directly and rapidly in predicting changes in contrast within an area of suspected pathology, may be applied using other scanning combinations and procedures.